1. Field of Invention
This invention is directed to implementing a feedback control loop for correcting non-uniform banding print quality defect. This invention is also directed to using array sensors and other point sensors for measuring banding and transfer efficiency in printing operations.
2. Description of Related Art
A common image quality defect introduced by the copying or printing process is banding. Banding generally refers to periodic defects on an image caused by a one-dimensional density variation in the process (slow scan) directions. An example of this kind of image quality defect, or periodic banding, is illustrated in FIG. 1. Bands can result due to many xerographic subsystem defects. Examples of these defects are run-out in the developer roll or photoreceptor drum, wobble in the polygon mirror of the laser raster optical scanner (ROS), and periodic variations in the photoreceptor motion, and the like. The sensitivity of print quality to these parameters can also depend on other factors. For example, the sensitivity of print quality to developer roll run-out depends largely on the age of the developer in semiconductive magnetic brush development. The problem of banding defect is generally addressed by focusing on mechanical design such as, for instance, maintaining tight tolerances on developer roll run-out, open loop operation, and the like.
Feedback controls were also introduced as a means to mitigate banding. Using a feedback control approach enables the use of components with relaxed tolerances, which would reduce unit machine cost (UMC). Also, controller design could be easily scaled from one product to the next. Moreover, feedback control is inherently robust to subsystem variations, such as developer material variations. The key shortcoming of this approach is that the banding defects are assumed to be uniform in the cross-process direction, as illustrated in FIG. 1.
However, banding is generally not uniform in the cross-process direction. In particular, developer roll run-out can give rise to banding that is not uniform. FIG. 2 illustrates typical profiles of developer roll run-out, and FIG. 3 shows examples of non-uniform banding associated with these roll run-out profiles. In FIG. 3, X refers to the cross-process direction and Y refers to the process direction. In the case of uniform banding, density variations are only a periodic function of the process direction position Y. That is, for a fixed value of Y, the density is constant in the X-direction, i.e., the cross-process direction. However, this case would only occur if the developer roll was only out of round, i.e., was not perfectly round, as illustrated in FIG. 3a. In the case of non-uniform banding, density variations are not only periodic in the process direction Y, but are a function of the cross-process direction X as well. For instance, banding due to bowing, and to the combination of both conicity and roundness are examples of non-uniform banding, and are illustrated in FIGS. 3b and 3c, respectively. For these banding examples, the density variations in the X-direction for a fixed Y position are qualitatively shown in FIG. 4, which relates developed mass average (DMA) with respect to the cross-process direction X. For both uniform and non-uniform banding, a typical density variation in the process direction Y, for a fixed X-coordinate, is shown in FIG. 5.
Another problem occurring in print and copy operations is high frequency banding. High frequency banding is a periodic modulation of a print with closely spaced peaks and troughs that run in the process direction. The peaks and troughs are so closely spaced that toner area coverage sensors using an illumination spot of a few millimeters in diameter cannot resolve the peaks and troughs. A primary cause of high frequency banding is, for instance, defect in the laser Raster Optical Scanner (ROS). These defects might include wobble in the ROS polygon mirror as it rotates, variations in the facet reflectivity, or errors in alignment of multibeam ROS's. Other subsystems, such as wire vibration in hybrid scavengeless development, may also contribute to high frequency banding. Accordingly, elimination of these defects has required manufacturing these systems and subsystems to high precision and at higher costs.
Another problem associated with print quality in print and copy operations is incomplete transfer of the toner image from the photoreceptor or from the intermediate belt to the paper. Because of some strongly adhering toners to the photoreceptor, low charge toner, air breakdown, or other reason, the transfer of the image from the photoreceptor to the intermediate transfer belt or paper, or from the intermediate transfer belt to the paper, will be incomplete. If the efficiency of transfer of the toner varies significantly from 100%, the density of toner on the final image may change. If the images are colored images, then changes in the density of toner will result in color shifts. Presently, printers are designed to have some latitude against variations in the external noises that cause transfer failures and these designs come at some cost.
An alternative approach, if the change in transfer efficiency can be detected before any image quality change occurs, is to adjust transfer subsystems set points to maintain a high transfer efficiency. Generally, the transfer efficiency can constantly be monitored in order to control the transfer efficiencies throughout and regardless of the various noises occurring in the xerographic process. However, to implement this approach, a sensitive measure of the toner residual mass must be made. Currently, a conventional sensor of toner mass on a photoreceptor is generally a toner area coverage (TAC) sensor. The TAC sensor monitors the change in the reflected light that the presence of toner on a photoreceptor causes. However, the TAC sensor is not accurate at low mass coverages. The background signal of the photoreceptor undergoes drifting due to, for example, the structure of the photoreceptor surface, variations in the illumination source, contaminants on the photoreceptor, and other noise sources. This drifting can dominate any small change the presence of a low area coverage of residual mass may cause, which may cause the low area coverage to remain undetected.
The detection of toner at very low coverages, such as for example of coverages smaller than 0.005 mg/cm2, can be important in diagnosing failures in the xerographic process. Accordingly, a technique for detecting low levels of toner is particle counting. This technique consists in submitting a small region of the surface of the photoreceptor to a microscope at a magnification such that the toner particles can be resolved. The number of toner particles over a given area is counted, either manually or automatically with a digital processing software, and the mass of toner present on the surface is inferred from the known density of the toner and the size of the toner particles. However, this technique is time-consuming and cannot be incorporated into the control system of a printer.